Viscoelastic Behavior of Sheep Respiratory System During Mechanical Ventilation

Background: Sheep are increasingly used as experimental animal model for respiratory diseases, however, respiratory mechanics of sheep is still incompletely characterized. Aim: To assess the viscoelastic behavior of the respiratory system, chest wall and lung in the whole inspiratory capacity range using the technique of rapid interruption during constant flow inflation. Methods: 6 healthy sheep (30-52 kg) were anesthetized, paralyzed and mechanically ventilated at 0, 5, 10 and 15 cmH2O end-expiratory pressure (ZEEP, PEEP5, 10 and 15) with constant tidal volume (8 ml/kg) and variable inspiratory flow (V̇insp). Viscoelastic resistance (Rvisc) and time-constant (τvisc) were measured by interpolating the relation between inspiratory duration (Ti) and additional resistance (ΔR) at each PEEP level with the equation ΔR=Rvisc(1-exp(-Ti/τvisc)). ΔR is the slow pressure drop observed during a 5s post-inspiratory hold divided by V̇insp, and reflects stress-relaxation. Esophageal pressure recording allowed to partition the viscoelastic properties of the respiratory system (rs) into those of chest wall (cw) and lung (L). Results: At ZEEP, PEEP5, 10 and 15, end-inspiratory volume was 44±8, 61±8, 79±7 and 92±5 % of vital capacity. Rvisc,cw, τvisc,rs, τvisc,cw and τvisc,L did not change with volume (P=0.95, 0.10, 0.69 and 0.20, respectively), while Rvisc,L and thus Rvisc,rs increased from ZEEP (5.3±2.0 and 9.9±1.7 cmH2Os/L, respectively) to PEEP15 (17.9±6.0 and 26.1±9.3 cmH2Os/L, P<0.01 and 0.02, respectively). Conclusions: Sheep lungs show a volume-dependent viscoelastic behavior. Quantitatively, the contribution of viscoelastic pressure is relevant, amounting to 10-15% of total inspiratory pressure at end-inflation.